1. Field of the Invention
The present invention is directed to shale shakers with vibrating screens, and, in one aspect, to shale shakers with tiered screens.
2. Description of Related Art
The prior art discloses a wide variety of vibrating screens, devices which use them, shale shakers, and screens for shale shakers. In shale shakers which use a plurality of screens, problems arise when an initial screen encounters such a large amount of fluid, e.g. drilling mud, that adequate and efficient screening is not possible.
The need for solids control in drilling mud in hydrocarbon well drilling is well known in the prior art. Drilling mud, typically a mixture of clay and water and various additives, is pumped through a hollow drill string (pipe, drill collar, bit, etc.) down into a well and exits through holes in a drillbit. The mud picks up cuttings (rock bits) and other solids from the well and carries them upwardly away from the bit and out of the well in a space between the well walls and the drill string. At the top of the well, the solids-laden mud is introduced to a shale shaker, a device which typically has a series of screens arranged in tiered or flat disposition with respect to each other. The screens catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the mud used during the drilling operation, recirculation of the drilled solids can create viscosity and gel problems in the mud, as well as increasing wear in mud pumps and other mechanical equipment used for drilling. In some shale shakers a fine screen cloth is used with the vibrating screen. The screen may have two or more overlying layers of screen cloth. The frame of the vibrating screen is suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism, e.g. an unbalanced weight on a rotating shaft connected to the frame. Each screen may be vibrated by vibratory equipment to create a flow of trapped solids toward an end of the screen on a top surface of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed.
In certain prior art shale shakers with one (or more) processing screens, a relatively small additional modular secondary screen is disposed at an end screen's discharge to provide for additional drying of solids to be discharged from the shaker. Such additional screens cannot adequately deal with a surge in fluid flow, e.g. during a "bottoms up" or riser pipe circulation condition. Also, such additional screens require inventorying of different screens. In other prior art systems, the discharge of one or more shale shakers is fed to another shale shaker for further de-liquefying and de-oiling. Such a process necessarily requires at least two shale shakers.